Abstract Early pregnancy loss affects 15-20% of implantation-confirmed pregnancies. Majority of these pregnancy losses occur during first trimester and defective development of trophoblast progenitors, which assures embryo implantation and placentation, is one of the leading causes for early pregnancy loss. However, we have a poor understanding of molecular mechanisms that regulate trophoblast progenitor self-renewal, differentiation and function in early postimplantation embryos. Our published and preliminary studies establish that hippo signaling effector, transcription factor TEAD4 is conserved in trophoblast progenitors across mammalian species and is a critical regulator to specify and maintain the trophoblast cell lineage during early mammalian development. The overarching goal of this proposal is to further define TEAD4-mediated conserved molecular mechanisms that are specifically involved in regulating trophoblast progenitor self-renewal and differentiation during post-implantation placenta development. In addition, we will also test whether alteration of those mechanisms is associated with recurrent pregnancy loss (RPL). Three specific aims are proposed. Aim 1 will study mutant mouse models to test the hypothesis that TEAD4 regulates trophoblast stem-like progenitor cell (TSPC) self-renewal in early postimplantation embryos. Aim 2 will test the hypothesis that cell-autonomous function of TEAD4 in lineage-specific trophoblast progenitors ensures proper development of differentiated trophoblast cells and formation of the maternal-fetal interface. In Aim 3, we will test functional importance of TEAD4 in human primary cytotrophoblasts (CTBs) and CTB- derived human trophoblast stem cells (TSCs). We will also recruit patients with known history of recurrent pregnancy loss (RPL) to isolate CTBs and establish patient-specific TSCs. The goal is to test the hypothesis that TEAD4 regulates self-renewal of CTB progenitors in a developing human placenta and defective function of TEAD4 is the molecular cause for a subset of unexplained (idiopathic) RPLs.